1. Field of the Invention
RF Transmission systems are used to transmit RF signals from point to point, for example, from an antenna to a transceiver or the like. Common forms of RF transmission systems include coaxial cables and striplines.
2. Description of Related Art
Prior coaxial cables typically have a coaxial configuration with a circular outer conductor evenly spaced away from a circular inner conductor by a dielectric support such as polyethylene foam or the like. The electrical properties of the dielectric support and spacing between the inner and outer conductor define a characteristic impedance of the coaxial cable. Circumferential uniformity of the spacing between the inner and outer conductor prevents introduction of impedance discontinuities into the coaxial cable that would otherwise degrade electrical performance.
An industry standard characteristic impedance is 50 ohms. Coaxial cables configured for 50 ohm characteristic impedance generally have an increased inner conductor diameter compared to higher characteristic impedance coaxial cables such that the metal inner conductor material cost is a significant portion of the entire cost of the resulting coaxial cable. To minimize material costs, the inner and outer conductors may be configured as thin metal layers for which structural support is then provided by less expensive materials. For example, commonly owned U.S. Pat. No. 6,800,809, titled “Coaxial Cable and Method of Making Same”, by Moe et al, issued Oct. 5, 2004, hereby incorporated by reference in the entirety, discloses a coaxial cable structure wherein the inner conductor is formed by applying a metallic strip around a cylindrical filler and support structure comprising a cylindrical plastic rod support structure with a foamed dielectric layer therearound. The resulting inner conductor structure has significant materials cost and weight savings compared to coaxial cables utilizing solid metal inner conductors. However, these structures can incur additional manufacturing costs, due to the multiple additional manufacturing steps required to sequentially apply each layer of the structure.
One limitation with respect to metal conductors and/or structural supports replacing solid metal conductors is bend radius. Generally, a larger diameter coaxial cable will have a reduced bend radius before the coaxial cable is distorted and/or buckled by bending. In particular, structures may buckle and/or be displaced out of coaxial alignment by cable bending in excess of the allowed bend radius, resulting in cable collapse and/or degraded electrical performance.
A further cable consideration is supporting and securing the cable along its length, for example as the cable is routed and secured along a radio tower. Prior cables configured for hanging in the air, such as telephone and/or CATV cables between utility poles and a residence, have been configured with encapsulated messenger wires to increase the strength of the cable and/or provide a sturdy attachment point separate from the signal conductor portion of the cable. Thereby, the cable strength is improved and the cable may be secured with reduced risk of damage to the signal conductor portion of the cable. However, messenger wires may increase the materials cost and overall weight of the cable.
A stripline is a flat conductor sandwiched between parallel interconnected ground planes. Striplines have the advantage of being non-dispersive and may be utilized for transmitting high frequency RF signals. Striplines may be cost effectively generated using printed circuit board technology or the like. However, striplines may be expensive to manufacture in longer lengths/larger dimensions. Further, where a solid stacked printed circuit board type stripline structure is not utilized, the conductor sandwich is generally not self supporting and/or aligning, compared to a coaxial cable, and as such may require significant additional support/reinforcing structure.
Competition within the RF cable industry has focused attention upon reducing materials and manufacturing costs, electrical characteristic uniformity, defect reduction, installation simplification and overall improved manufacturing quality control.
Therefore, it is an object of the invention to provide a coaxial cable and method of manufacture that overcomes deficiencies in such prior art.